clean up sfepy/mechanics/units.py
clean up sfepy/mechanics/units.py

lokik/sfepy,rc/sfepy,lokik/sfepy,vlukes/sfepy,vlukes/sfepy,BubuLK/sfepy,sfepy/sfepy,sfepy/sfepy,sfepy/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,rc/sfepy,lokik/sfepy,lokik/sfepy,BubuLK/sfepy